They can't support it on XP - DX10 is not a strict following from DX9c. The API is completely rewritten from scratch to take advantage of a new driver model/system that is not supported by XP.

And besides - any frames above 85fps are discarded because that is the maximum refresh limit of many monitors. And your eye can't really see any difference beyond 65-75fps depending on lighting, angle and other factors.

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"A lie travels half-way around the world while the truth is still putting on its boots."
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The driver support model is completely different from XP to Vista. Not only is DX10 completely rewritten - the entire backend of Vista is completely new from XP. It would take more than just a few driver updates to get it working on XP because DX is directly integrated into the OS.

We're not just talking of a few tweaks - we're talking about a rewrite of the entire way XP handles graphics drivers and their hardware. That's not exactly an easy task on an OS which has already defined how it handles drivers. This means XP would have to have two different ways to handle drivers - and this would introduce all sorts of problems of hardware conflicting with applications. It would introduce confusion for programmers and users and would make the OS buggier.

Microsoft was smart to make a clean break from XP's code and introduce a newly rewritten API on a new system. It works much better that way.

__________________
"A lie travels half-way around the world while the truth is still putting on its boots."
-Mark Twain

Why wouldn't they? Vista was designed to be backwards compatible as per Microsoft's own technical summaries.

Vista has application compatibility layers as part of its layered software model in much the same way XP had layered compatibility architecture to run Win95 and 98 programs. The layer produces a slight hit in performance but only applies to pre-Vista application (a driver is an application) and the hit is negligible anyway. If you want an example of an application compatibility layer think of WINE for Linux which can run Win32 apps (and no, WINE is not an emulator).

There are a few drivers though that have been confirmed to have issues.

__________________
"A lie travels half-way around the world while the truth is still putting on its boots."
-Mark Twain